The present invention relates to steering systems for forklift trucks and, more particularly, to a steering system in which an optical sensor is positioned adjacent the steering column to detect a steering demand and signal the hydraulic pump motor to increase speed in response to the demand.
In order to ensure that the steering motor always has a sufficient supply of hydraulic fluid when there is a steering demand, it is usual to use a priority valve. Priority valves suitable for use with standard load sensing steering systems have been manufactured by suppliers such as Danfoss (OLS series). These valves are variable priority distribution valves with a single inlet P from the pump and two outlets, CF which supplies a steer unit and EF which supplies the working hydraulics. At low pump flow, all of the pump flow is directed to the CF output, and at higher pump flow a priority flow sufficient to drive the steering is directed to the CF output with the remaining flow being directed to the EF output. The position of the valve is typically controlled by a load sensing (LS) signal derived from the steer unit.
Unless the pump is always run at a sufficient speed to generate at least the flow and pressure required for operation of the steer unit, which is clearly an inefficient option, it is also necessary to speed up or start the motor in response to the initiation of a steering demand, in order to ensure that sufficient hydraulic flow and pressure can be provided to the steer unit. It is standard practice to use a pulse control unit as a motor controller in order to vary the speed of the motor, and therefore of the pump, in order to control its output. In European Patent No. 0251290 (Still GmbH), it is proposed to use the load sensing signal from the steer unit to convert into an electrical signal to input to the motor controller in addition to the usual demand signals from the working hydraulics.
In order to convert a pressure signal to an electrical signal, it is necessary to use a device such as a pressure switch. Such pressure switches can be unreliable and are relatively expensive. Moreover, it is necessary to set a pressure switch to a predetermined level which will prevent the motor being activated unnecessarily, while ensuring that response to an actual steering need is as rapid as possible which is essential particularly in emergency situations.
In most systems which use a single motor and pump for both working and steering hydraulics, it is usual for the motor to be stationary when there is no demand for working fluid from either system. Such arrangements are described in U.K. Patent No. 1565639 (Towmotor) and European Patent No. 0283803 (Steinbock Boss). This can create significant steering response problems when steering demand occurs when the motor is stationary. A significant time delay can occur after rotation of a steering wheel while the pressure in a static load sensing line increases to a sufficient value to activate the pressure switch, and thereby control the motor so that it is switched on. Such delays can be unacceptable in emergency situations.
Accordingly, there is a need for a steering system that provides rapid steering response to a steering demand, yet is more efficient than continuously operating the pump motor at a speed sufficient to meet a demand.